Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $166.37   8.192%   $179.999   $180.00   $180.00   $180.00  
  Purchase #2 $102.62   10.816%   $113.7194   $113.72   $113.70   $113.70  
  Purchase #3 $27.48   12.259%   $30.8488   $30.85   $30.85   $30.80  
  Purchase #4 $111.55   5.550%   $117.741   $117.74   $117.75   $117.70  
  Purchase #5 $70.71   12.158%   $79.3069   $79.31   $79.30   $79.30  
  Purchase #6 $58.35   6.128%   $61.9257   $61.93   $61.95   $61.90  
  Purchase #7 $77.80   10.568%   $86.0219   $86.02   $86.00   $86.00  
  7 purchase totals = $614.88       $669.5627   $669.57   $669.55   $669.40